This invention is closely related to and constitutes an improvement upon U.S. Pat. No. 3,245,067, which describes and claims an early warning fire detector characterized by a pair of impedance means or resistance grids sensitive to the increase in electrical conductivity of the air upon initiation of a fire, the grids being connected in a bridge circuit and comprising a first grid freely exposed to the atmosphere and a second grid accommodating only slow or delayed access thereto of the atmosphere, whereby to provide a rate-of-change device that senses the fast change in the electrical conductivity of the atmosphere that occurs even in the incipient stages of fire, as well as in the smoke, flame and heat stages of fire. The second grid compensates the first or detector grid against slowly occurring changes, such as atmospheric changes and those due to one or more people smoking in the room, but does not respond with sufficient rapidity to the fast change caused by a fire, with the result that the bridge becomes unbalanced and a signal is transmitted via an suitable to suiable alarm means.
The grids each comprise a substrate of known surface resistivity and an electrode configuration in the form of two opposing and interfitting comblike electrodes on the substrate, one comprising the positive plate and the other the negative. Power is supplied to the positive plate of one grid and the negative of the other; the negative of the one grid and the positive of the other being connected and the junction constituting the output of a bridge which is coupled to the amplifier and the alarm or other suitable control devices.